The present invention relates to a sound transducer structure and to a method for manufacturing it and, in particular, to how different sound transducer structures can be manufactured and how geometries and characteristics of the sound transducer structures can be adjusted to fulfill different requirements to the sound transducer structures.
Sound transducer structures are used in a plurality of applications, such as, for example, in microphones or loudspeakers, these two principally only differing in that in microphones sound energy is converted to electric energy and in loudspeakers electric energy is converted to sound energy. Since sound transducers detect or generate dynamic pressure changes, the invention also relates to pressure sensors.
In general, sound transducers, such as, for example, microphones, are to be manufacturable at low cost and be as small as possible. Due to these requirements, microphones and sound transducers are often produced in silicon technology, wherein due to the different desired fields of application and sensitivities, there are a plurality of potential configurations of sound transducers each comprising different geometrical configurations. Microphones, for example, may be based on the principle of measuring a capacity. A movable membrane which is deformed or deflected by pressure changes is arranged in a suitable distance to a counter electrode such that a change in capacity resulting from a deformation or deflection of the membrane between the membrane and the counter electrode may be used to draw conclusions as to pressure or sound changes. Such a structure is typically operated by a bias voltage, i.e. a potential which may be adjusted freely to the respective circumstances is applied between the membrane and the counter electrode.
Other parameters determining the sensitivity of such a microphone or the signal-to-noise ratio (SNR) of the microphone are, for example, rigidity of the membrane, diameter of the membrane or rigidity of the counter electrode which may also deform under the influence of the electrostatic force between the membrane and the counter electrode. Different possibilities result depending on the profile of requirements (for a finished processed sound transducer), such as, for example, a combination of low a desired operating voltage with medium mechanical sensitivity, a combination of low an operating voltage with high mechanical sensitivity or a combination of high an operating voltage with medium mechanical sensitivity.
In addition to the mechanical characteristic of the materials used, particularly high a requirement is often made as to the manufacturing tolerance of the membrane diameter or membrane dimension which has considerable influence on the characteristics of a microphone. This will be of particular relevance if several microphones are to be used in an array and consequently must have characteristics as identical as possible. Often, a microphone chip the membrane of which is accessible from both sides is glued onto a substrate in a sound-proof manner. Thus, a back volume forming a cavity is sealed by one side of the membrane. The characteristics of the cavity formed are decisive for the sensitivity and the SNR of the microphone since the cavity counteracts the deflection or deformation of the membrane and can attenuate this movement since the membrane in a sense has to act against a volume of a certain “viscosity”. The diameter of the membrane in relation to the cavity volume given plays an important role for a quantitative estimation of this effect.
Considering the plurality of elements possible and the plurality of parameters, the problem arising often is that production lines by means of which it is possible to manufacture the most different sound transducer structures have to be provided.